The present invention relates to an analog to digital converter and, more particularly, to a converter and method of converting an optical analog signal to an optical digital signal.
Because of the increased use of optical sensors and optical fibers for the high-bandwidth, noise-immune transmission of signals, analog signals are more often being provided optically at increasingly faster data rates in excess of 3.0 GHz (3,000 MHz) for data words having a length exceeding 10 bits. High performance analog to digital (hereinafter "A/D") converters are required to convert this high-speed data on a real time basis, but such converters currently operate at rates no better than a few hundred MHz for digital words in the four-bit range and tens of MHz for digital words in the ten-bit range. Three problems arise as a result of the unavailability of an A/D converter that operates at such high speeds.
First, systems using high speed transducers, such as radar and sonar, require analog preprocessing equipment to interface the sensor and the digital equipment so that the data rate is reduced to the range of the A/D converter. This analog preprocessing equipment is expensive and more difficult to operate than digital processing equipment. Second, existing A/D converters will not provide a digital output at a rate sufficiently large to drive optical devices currently under development that operate at rates up to 30 GHz. Third, current A/D converters have a limited dynamic range which makes the converter susceptible to noise interference so that systems such as radar and sonar can be easily jammed.
The A/D converters currently under development use electronics or other technologies rather than optics to accomplish high speed A/D conversion. For example, electronic A/D converters, such as flash converters, have been used, but are limited in bit size because encoders are required to provide an interface between the comparator outputs and the digital output of the flash converter. The encoders increase in size, cost and delay as the number of bits is increased. Other technologies having similar problems include electro-optic waveguide A/D converters and liquid crystal A/D converters. Accordingly, there is a need for an A/D converter capable of operating at high data speeds for word lengths over ten bits and, more specifically, one that is capable of converting an optical analog signal directly to an optical digital signal.